[PATCH] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[PATCH] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[Jens Axboe]

We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
dereference. Park the SQPOLL thread while getting the task cpu and pid for
fdinfo, this ensures we have a stable view of it.

Cc: stable@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
index c53678875416..cd2a0c6b97c4 100644
--- a/io_uring/fdinfo.c
+++ b/io_uring/fdinfo.c
@@ -53,7 +53,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
 __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
 {
 	struct io_ring_ctx *ctx = f->private_data;
-	struct io_sq_data *sq = NULL;
 	struct io_overflow_cqe *ocqe;
 	struct io_rings *r = ctx->rings;
 	unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
@@ -64,6 +63,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
 	unsigned int cq_shift = 0;
 	unsigned int sq_shift = 0;
 	unsigned int sq_entries, cq_entries;
+	int sq_pid = -1, sq_cpu = -1;
 	bool has_lock;
 	unsigned int i;
 
@@ -143,13 +143,18 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
 	has_lock = mutex_trylock(&ctx->uring_lock);
 
 	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
-		sq = ctx->sq_data;
-		if (!sq->thread)
-			sq = NULL;
+		struct io_sq_data *sq = ctx->sq_data;
+
+		io_sq_thread_park(sq);
+		if (sq->thread) {
+			sq_pid = task_pid_nr(sq->thread);
+			sq_cpu = task_cpu(sq->thread);
+		}
+		io_sq_thread_unpark(sq);
 	}
 
-	seq_printf(m, "SqThread:\t%d\n", sq ? task_pid_nr(sq->thread) : -1);
-	seq_printf(m, "SqThreadCpu:\t%d\n", sq ? task_cpu(sq->thread) : -1);
+	seq_printf(m, "SqThread:\t%d\n", sq_pid);
+	seq_printf(m, "SqThreadCpu:\t%d\n", sq_cpu);
 	seq_printf(m, "UserFiles:\t%u\n", ctx->nr_user_files);
 	for (i = 0; has_lock && i < ctx->nr_user_files; i++) {
 		struct file *f = io_file_from_index(&ctx->file_table, i);

-- 
Jens Axboe

Re: [PATCH] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[Jens Axboe]

On 10/23/23 9:17 AM, Gabriel Krisman Bertazi wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
>> We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
>> dereference. Park the SQPOLL thread while getting the task cpu and pid for
>> fdinfo, this ensures we have a stable view of it.
>>
>> Cc: stable@vger.kernel.org
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
>> index c53678875416..cd2a0c6b97c4 100644
>> --- a/io_uring/fdinfo.c
>> +++ b/io_uring/fdinfo.c
>> @@ -53,7 +53,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
>>  __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>  {
>>  	struct io_ring_ctx *ctx = f->private_data;
>> -	struct io_sq_data *sq = NULL;
>>  	struct io_overflow_cqe *ocqe;
>>  	struct io_rings *r = ctx->rings;
>>  	unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
>> @@ -64,6 +63,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>  	unsigned int cq_shift = 0;
>>  	unsigned int sq_shift = 0;
>>  	unsigned int sq_entries, cq_entries;
>> +	int sq_pid = -1, sq_cpu = -1;
>>  	bool has_lock;
>>  	unsigned int i;
>>  
>> @@ -143,13 +143,18 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>  	has_lock = mutex_trylock(&ctx->uring_lock);
>>  
>>  	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
>> -		sq = ctx->sq_data;
>> -		if (!sq->thread)
>> -			sq = NULL;
>> +		struct io_sq_data *sq = ctx->sq_data;
>> +
>> +		io_sq_thread_park(sq);
>> +		if (sq->thread) {
>> +			sq_pid = task_pid_nr(sq->thread);
>> +			sq_cpu = task_cpu(sq->thread);
>> +		}
>> +		io_sq_thread_unpark(sq);
> 
> Jens,
> 
> io_sq_thread_park will try to wake the sqpoll, which is, at least,
> unnecessary. But I'm thinking we don't want to expose the ability to
> schedule the sqpoll from procfs, which can be done by any unrelated
> process.
> 
> To solve the bug, it should be enough to synchronize directly on
> sqd->lock, preventing sq->thread from going away inside the if leg.
> Granted, it is might take longer if the sqpoll is busy, but reading
> fdinfo is not supposed to be fast.  Alternatively, don't call
> wake_process in this case?

I did think about that but just went with the exported API. But you are
right, it's a bit annoying that it'd also wake the thread, in case it
was idle. Probably mostly cosmetic, but we may as well just stick with
grabbing the sqd mutex. I'll send a v2.

-- 
Jens Axboe

Re: [PATCH] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[Pavel Begunkov]

On 10/23/23 16:27, Jens Axboe wrote:
> On 10/23/23 9:17 AM, Gabriel Krisman Bertazi wrote:
>> Jens Axboe <axboe@kernel.dk> writes:
>>
>>> We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
>>> dereference. Park the SQPOLL thread while getting the task cpu and pid for
>>> fdinfo, this ensures we have a stable view of it.
>>>
>>> Cc: stable@vger.kernel.org
>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>
>>> ---
>>>
>>> diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
>>> index c53678875416..cd2a0c6b97c4 100644
>>> --- a/io_uring/fdinfo.c
>>> +++ b/io_uring/fdinfo.c
>>> @@ -53,7 +53,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
>>>   __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>   {
>>>   	struct io_ring_ctx *ctx = f->private_data;
>>> -	struct io_sq_data *sq = NULL;
>>>   	struct io_overflow_cqe *ocqe;
>>>   	struct io_rings *r = ctx->rings;
>>>   	unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
>>> @@ -64,6 +63,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>   	unsigned int cq_shift = 0;
>>>   	unsigned int sq_shift = 0;
>>>   	unsigned int sq_entries, cq_entries;
>>> +	int sq_pid = -1, sq_cpu = -1;
>>>   	bool has_lock;
>>>   	unsigned int i;
>>>   
>>> @@ -143,13 +143,18 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>   	has_lock = mutex_trylock(&ctx->uring_lock);
>>>   
>>>   	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
>>> -		sq = ctx->sq_data;
>>> -		if (!sq->thread)
>>> -			sq = NULL;
>>> +		struct io_sq_data *sq = ctx->sq_data;
>>> +
>>> +		io_sq_thread_park(sq);
>>> +		if (sq->thread) {
>>> +			sq_pid = task_pid_nr(sq->thread);
>>> +			sq_cpu = task_cpu(sq->thread);
>>> +		}
>>> +		io_sq_thread_unpark(sq);
>>
>> Jens,
>>
>> io_sq_thread_park will try to wake the sqpoll, which is, at least,
>> unnecessary. But I'm thinking we don't want to expose the ability to
>> schedule the sqpoll from procfs, which can be done by any unrelated
>> process.
>>
>> To solve the bug, it should be enough to synchronize directly on
>> sqd->lock, preventing sq->thread from going away inside the if leg.
>> Granted, it is might take longer if the sqpoll is busy, but reading
>> fdinfo is not supposed to be fast.  Alternatively, don't call
>> wake_process in this case?
> 
> I did think about that but just went with the exported API. But you are
> right, it's a bit annoying that it'd also wake the thread, in case it

Waking it up is not a problem but without parking sq thread won't drop
the lock until it's time to sleep, which might be pretty long leaving
the /proc read stuck on the lock uninterruptibly.

Aside from parking vs lock, there is a lock inversion now:

proc read                   | SQPOLL
                             |
try_lock(ring) // success   |
                             | woken up
                             | lock(sqd); // success
lock(sqd); // stuck         |
                             | try to submit requests
                             | -- lock(ring); // stuck


> was idle. Probably mostly cosmetic, but we may as well just stick with
> grabbing the sqd mutex. I'll send a v2.


-- 
Pavel Begunkov

Re: [PATCH] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[Jens Axboe]

On 10/25/23 6:09 AM, Pavel Begunkov wrote:
> On 10/23/23 16:27, Jens Axboe wrote:
>> On 10/23/23 9:17 AM, Gabriel Krisman Bertazi wrote:
>>> Jens Axboe <axboe@kernel.dk> writes:
>>>
>>>> We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
>>>> dereference. Park the SQPOLL thread while getting the task cpu and pid for
>>>> fdinfo, this ensures we have a stable view of it.
>>>>
>>>> Cc: stable@vger.kernel.org
>>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>
>>>> ---
>>>>
>>>> diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
>>>> index c53678875416..cd2a0c6b97c4 100644
>>>> --- a/io_uring/fdinfo.c
>>>> +++ b/io_uring/fdinfo.c
>>>> @@ -53,7 +53,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
>>>>   __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>   {
>>>>       struct io_ring_ctx *ctx = f->private_data;
>>>> -    struct io_sq_data *sq = NULL;
>>>>       struct io_overflow_cqe *ocqe;
>>>>       struct io_rings *r = ctx->rings;
>>>>       unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
>>>> @@ -64,6 +63,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>       unsigned int cq_shift = 0;
>>>>       unsigned int sq_shift = 0;
>>>>       unsigned int sq_entries, cq_entries;
>>>> +    int sq_pid = -1, sq_cpu = -1;
>>>>       bool has_lock;
>>>>       unsigned int i;
>>>>   @@ -143,13 +143,18 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>       has_lock = mutex_trylock(&ctx->uring_lock);
>>>>         if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
>>>> -        sq = ctx->sq_data;
>>>> -        if (!sq->thread)
>>>> -            sq = NULL;
>>>> +        struct io_sq_data *sq = ctx->sq_data;
>>>> +
>>>> +        io_sq_thread_park(sq);
>>>> +        if (sq->thread) {
>>>> +            sq_pid = task_pid_nr(sq->thread);
>>>> +            sq_cpu = task_cpu(sq->thread);
>>>> +        }
>>>> +        io_sq_thread_unpark(sq);
>>>
>>> Jens,
>>>
>>> io_sq_thread_park will try to wake the sqpoll, which is, at least,
>>> unnecessary. But I'm thinking we don't want to expose the ability to
>>> schedule the sqpoll from procfs, which can be done by any unrelated
>>> process.
>>>
>>> To solve the bug, it should be enough to synchronize directly on
>>> sqd->lock, preventing sq->thread from going away inside the if leg.
>>> Granted, it is might take longer if the sqpoll is busy, but reading
>>> fdinfo is not supposed to be fast.  Alternatively, don't call
>>> wake_process in this case?
>>
>> I did think about that but just went with the exported API. But you are
>> right, it's a bit annoying that it'd also wake the thread, in case it
> 
> Waking it up is not a problem but without parking sq thread won't drop
> the lock until it's time to sleep, which might be pretty long leaving
> the /proc read stuck on the lock uninterruptibly.
> 
> Aside from parking vs lock, there is a lock inversion now:
> 
> proc read                   | SQPOLL
>                             |
> try_lock(ring) // success   |
>                             | woken up
>                             | lock(sqd); // success
> lock(sqd); // stuck         |
>                             | try to submit requests
>                             | -- lock(ring); // stuck

Yeah good point, forgot we nest these opposite of what you'd expect.
Honestly I think the fix here is just to turn it into a trylock. Yes
that'll miss some cases where we could've gotten the pid/cpu, but
doesn't seem worth caring about.

IOW, fold in this incremental.

diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
index af1bdcc0703e..f04a43044d91 100644
--- a/io_uring/fdinfo.c
+++ b/io_uring/fdinfo.c
@@ -145,12 +145,13 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
 	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
 		struct io_sq_data *sq = ctx->sq_data;
 
-		mutex_lock(&sq->lock);
-		if (sq->thread) {
-			sq_pid = task_pid_nr(sq->thread);
-			sq_cpu = task_cpu(sq->thread);
+		if (mutex_trylock(&sq->lock)) {
+			if (sq->thread) {
+				sq_pid = task_pid_nr(sq->thread);
+				sq_cpu = task_cpu(sq->thread);
+			}
+			mutex_unlock(&sq->lock);
 		}
-		mutex_unlock(&sq->lock);
 	}
 
 	seq_printf(m, "SqThread:\t%d\n", sq_pid);

-- 
Jens Axboe

Re: [PATCH] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[Pavel Begunkov]

On 10/25/23 14:44, Jens Axboe wrote:
> On 10/25/23 6:09 AM, Pavel Begunkov wrote:
>> On 10/23/23 16:27, Jens Axboe wrote:
>>> On 10/23/23 9:17 AM, Gabriel Krisman Bertazi wrote:
>>>> Jens Axboe <axboe@kernel.dk> writes:
>>>>
>>>>> We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
>>>>> dereference. Park the SQPOLL thread while getting the task cpu and pid for
>>>>> fdinfo, this ensures we have a stable view of it.
>>>>>
>>>>> Cc: stable@vger.kernel.org
>>>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>>
>>>>> ---
>>>>>
>>>>> diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
>>>>> index c53678875416..cd2a0c6b97c4 100644
>>>>> --- a/io_uring/fdinfo.c
>>>>> +++ b/io_uring/fdinfo.c
>>>>> @@ -53,7 +53,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
>>>>>    __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>    {
>>>>>        struct io_ring_ctx *ctx = f->private_data;
>>>>> -    struct io_sq_data *sq = NULL;
>>>>>        struct io_overflow_cqe *ocqe;
>>>>>        struct io_rings *r = ctx->rings;
>>>>>        unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
>>>>> @@ -64,6 +63,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>        unsigned int cq_shift = 0;
>>>>>        unsigned int sq_shift = 0;
>>>>>        unsigned int sq_entries, cq_entries;
>>>>> +    int sq_pid = -1, sq_cpu = -1;
>>>>>        bool has_lock;
>>>>>        unsigned int i;
>>>>>    @@ -143,13 +143,18 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>        has_lock = mutex_trylock(&ctx->uring_lock);
>>>>>          if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
>>>>> -        sq = ctx->sq_data;
>>>>> -        if (!sq->thread)
>>>>> -            sq = NULL;
>>>>> +        struct io_sq_data *sq = ctx->sq_data;
>>>>> +
>>>>> +        io_sq_thread_park(sq);
>>>>> +        if (sq->thread) {
>>>>> +            sq_pid = task_pid_nr(sq->thread);
>>>>> +            sq_cpu = task_cpu(sq->thread);
>>>>> +        }
>>>>> +        io_sq_thread_unpark(sq);
>>>>
>>>> Jens,
>>>>
>>>> io_sq_thread_park will try to wake the sqpoll, which is, at least,
>>>> unnecessary. But I'm thinking we don't want to expose the ability to
>>>> schedule the sqpoll from procfs, which can be done by any unrelated
>>>> process.
>>>>
>>>> To solve the bug, it should be enough to synchronize directly on
>>>> sqd->lock, preventing sq->thread from going away inside the if leg.
>>>> Granted, it is might take longer if the sqpoll is busy, but reading
>>>> fdinfo is not supposed to be fast.  Alternatively, don't call
>>>> wake_process in this case?
>>>
>>> I did think about that but just went with the exported API. But you are
>>> right, it's a bit annoying that it'd also wake the thread, in case it
>>
>> Waking it up is not a problem but without parking sq thread won't drop
>> the lock until it's time to sleep, which might be pretty long leaving
>> the /proc read stuck on the lock uninterruptibly.
>>
>> Aside from parking vs lock, there is a lock inversion now:
>>
>> proc read                   | SQPOLL
>>                              |
>> try_lock(ring) // success   |
>>                              | woken up
>>                              | lock(sqd); // success
>> lock(sqd); // stuck         |
>>                              | try to submit requests
>>                              | -- lock(ring); // stuck
> 
> Yeah good point, forgot we nest these opposite of what you'd expect.
> Honestly I think the fix here is just to turn it into a trylock. Yes
> that'll miss some cases where we could've gotten the pid/cpu, but
> doesn't seem worth caring about.
> 
> IOW, fold in this incremental.

Should work, otherwise you probably can just park first.

Long term it'd be nice to make sqpoll to not hold sqd->lock during
submission + polling as it currently does. Park callers sleep on the
lock, but if we replace it with some struct completion the rest
should be easy enough.


> diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
> index af1bdcc0703e..f04a43044d91 100644
> --- a/io_uring/fdinfo.c
> +++ b/io_uring/fdinfo.c
> @@ -145,12 +145,13 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>   	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
>   		struct io_sq_data *sq = ctx->sq_data;
>   
> -		mutex_lock(&sq->lock);
> -		if (sq->thread) {
> -			sq_pid = task_pid_nr(sq->thread);
> -			sq_cpu = task_cpu(sq->thread);
> +		if (mutex_trylock(&sq->lock)) {
> +			if (sq->thread) {
> +				sq_pid = task_pid_nr(sq->thread);
> +				sq_cpu = task_cpu(sq->thread);
> +			}
> +			mutex_unlock(&sq->lock);
>   		}
> -		mutex_unlock(&sq->lock);
>   	}
>   
>   	seq_printf(m, "SqThread:\t%d\n", sq_pid);
> 

-- 
Pavel Begunkov

Re: [PATCH] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[Jens Axboe]

On 10/25/23 8:09 AM, Pavel Begunkov wrote:
> On 10/25/23 14:44, Jens Axboe wrote:
>> On 10/25/23 6:09 AM, Pavel Begunkov wrote:
>>> On 10/23/23 16:27, Jens Axboe wrote:
>>>> On 10/23/23 9:17 AM, Gabriel Krisman Bertazi wrote:
>>>>> Jens Axboe <axboe@kernel.dk> writes:
>>>>>
>>>>>> We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
>>>>>> dereference. Park the SQPOLL thread while getting the task cpu and pid for
>>>>>> fdinfo, this ensures we have a stable view of it.
>>>>>>
>>>>>> Cc: stable@vger.kernel.org
>>>>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
>>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>>>
>>>>>> ---
>>>>>>
>>>>>> diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
>>>>>> index c53678875416..cd2a0c6b97c4 100644
>>>>>> --- a/io_uring/fdinfo.c
>>>>>> +++ b/io_uring/fdinfo.c
>>>>>> @@ -53,7 +53,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
>>>>>>    __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>>    {
>>>>>>        struct io_ring_ctx *ctx = f->private_data;
>>>>>> -    struct io_sq_data *sq = NULL;
>>>>>>        struct io_overflow_cqe *ocqe;
>>>>>>        struct io_rings *r = ctx->rings;
>>>>>>        unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
>>>>>> @@ -64,6 +63,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>>        unsigned int cq_shift = 0;
>>>>>>        unsigned int sq_shift = 0;
>>>>>>        unsigned int sq_entries, cq_entries;
>>>>>> +    int sq_pid = -1, sq_cpu = -1;
>>>>>>        bool has_lock;
>>>>>>        unsigned int i;
>>>>>>    @@ -143,13 +143,18 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
>>>>>>        has_lock = mutex_trylock(&ctx->uring_lock);
>>>>>>          if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
>>>>>> -        sq = ctx->sq_data;
>>>>>> -        if (!sq->thread)
>>>>>> -            sq = NULL;
>>>>>> +        struct io_sq_data *sq = ctx->sq_data;
>>>>>> +
>>>>>> +        io_sq_thread_park(sq);
>>>>>> +        if (sq->thread) {
>>>>>> +            sq_pid = task_pid_nr(sq->thread);
>>>>>> +            sq_cpu = task_cpu(sq->thread);
>>>>>> +        }
>>>>>> +        io_sq_thread_unpark(sq);
>>>>>
>>>>> Jens,
>>>>>
>>>>> io_sq_thread_park will try to wake the sqpoll, which is, at least,
>>>>> unnecessary. But I'm thinking we don't want to expose the ability to
>>>>> schedule the sqpoll from procfs, which can be done by any unrelated
>>>>> process.
>>>>>
>>>>> To solve the bug, it should be enough to synchronize directly on
>>>>> sqd->lock, preventing sq->thread from going away inside the if leg.
>>>>> Granted, it is might take longer if the sqpoll is busy, but reading
>>>>> fdinfo is not supposed to be fast.  Alternatively, don't call
>>>>> wake_process in this case?
>>>>
>>>> I did think about that but just went with the exported API. But you are
>>>> right, it's a bit annoying that it'd also wake the thread, in case it
>>>
>>> Waking it up is not a problem but without parking sq thread won't drop
>>> the lock until it's time to sleep, which might be pretty long leaving
>>> the /proc read stuck on the lock uninterruptibly.
>>>
>>> Aside from parking vs lock, there is a lock inversion now:
>>>
>>> proc read                   | SQPOLL
>>>                              |
>>> try_lock(ring) // success   |
>>>                              | woken up
>>>                              | lock(sqd); // success
>>> lock(sqd); // stuck         |
>>>                              | try to submit requests
>>>                              | -- lock(ring); // stuck
>>
>> Yeah good point, forgot we nest these opposite of what you'd expect.
>> Honestly I think the fix here is just to turn it into a trylock. Yes
>> that'll miss some cases where we could've gotten the pid/cpu, but
>> doesn't seem worth caring about.
>>
>> IOW, fold in this incremental.
> 
> Should work, otherwise you probably can just park first.

In general I think it's better to have the observational side be less of
an impact, which is why I liked not doing the parking. Sometimes apps do
stupid things and monitor fdinfo/ etc continually. As it's possible to
get the task/cpu anyway via other means, should be better to just skip
if we don't get the lock.

> Long term it'd be nice to make sqpoll to not hold sqd->lock during
> submission + polling as it currently does. Park callers sleep on the
> lock, but if we replace it with some struct completion the rest
> should be easy enough.

Yeah agree.

-- 
Jens Axboe