Re: [PATCH V2] futex: set FLAGS_HAS_TIMEOUT during demux for FUTEX_WAIT

[Darren Hart <dvhart@linux.intel.com>]

On 04/14/2011 11:56 AM, Eric Dumazet wrote:
> Le jeudi 14 avril 2011 à 11:45 -0700, Darren Hart a écrit :
>> The FLAGS_HAS_TIMEOUT flag was not getting set, causing the restart_block to
>> restart futex_wait() without a timeout after a signal.
>>
>> Commit b41277dc7a18ee332d in 2.6.38 introduced the regression by accidentally
>> removing the the FLAGS_HAS_TIMEOUT assignment from futex_wait() during the setup
>> of the restart block. It makes more sense to set the flag earlier during
>> do_futex() where the other flags are set, although futex_wait is the only user
>> of FLAGS_HAS_TIMEOUT as it is the only op using a relative timeout (an
>> unfortunately preexisting condition).
>>
>> Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=32922
>>
>> V2: Added references to commit message.
>>
>> Signed-off-by: Darren Hart <dvhart@linux.intel.com>
>> Reported-by: Tim Smith <tsmith201104@yahoo.com>
>> Reported-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Ingo Molnar <mingo@elte.hu>
>> CC: Eric Dumazet <eric.dumazet@gmail.com>
>> CC: John Kacur <jkacur@redhat.com>
>> ---
>>  kernel/futex.c |    3 +++
>>  1 files changed, 3 insertions(+), 0 deletions(-)
>>
>> diff --git a/kernel/futex.c b/kernel/futex.c
>> index bda4157..6eac6b6 100644
>> --- a/kernel/futex.c
>> +++ b/kernel/futex.c
>> @@ -2589,6 +2589,9 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
>>  			return -ENOSYS;
>>  	}
>>  
>> +	if (timeout)
>> +		flags |= FLAGS_HAS_TIMEOUT;
>> +
>>  	switch (cmd) {
>>  	case FUTEX_WAIT:
>>  		val3 = FUTEX_BITSET_MATCH_ANY;
> 
> 
> I dont agree with your fix. You add yet another test in futex fastpath.

I would say anything calling SYS_FUTEX is the futex slow path. The fast
path is cmpxchg in user space.

> 
> Prior commit  b41277dc7a18ee332d, this was done only in case of restart.

It was. My thinking was that it was inconsistent to have the
FLAGS_HAS_TIMEOUT only available if a signal was received and a restart
was required. This is the only place it is currently needed, but the
inconsistency concerns me.

> 
> Could we instead use 
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index dfb924f..fe28dc2 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1886,7 +1886,7 @@ retry:
>  	restart->futex.val = val;
>  	restart->futex.time = abs_time->tv64;
>  	restart->futex.bitset = bitset;
> -	restart->futex.flags = flags;
> +	restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;
>  
>  	ret = -ERESTART_RESTARTBLOCK;

How about the following, it reuses an existing if block and ensure the
FLAGS_HAS_TIMEOUT is always set if a timeout is used. It means the
FLAG_HAS_TIMEOUT is not available in the other futex_* routines with
timeouts (futex_lock_pi and futex_wait_requeue_pi), but they use absolute
timeouts and don't need it for restart - I can agree to that, although
I'm not keen on FLAG_HAS_TIMEOUT not being set whenever timeout is. That
could be added in the same way to the other functions if needed in the
future.


diff --git a/kernel/futex.c b/kernel/futex.c
index 6eac6b6..0a7eefd 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1846,6 +1846,7 @@ static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
 		hrtimer_init_sleeper(to, current);
 		hrtimer_set_expires_range_ns(&to->timer, *abs_time,
 					     current->timer_slack_ns);
+		flags |= FLAGS_HAS_TIMEOUT;
 	}
 
 retry:



-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel