linux-kernel - Re: [PATCH V2] futex: set FLAGS_HAS_TIMEOUT during demux for FUTEX

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Date:	Thu, 14 Apr 2011 20:56:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH V2] futex: set FLAGS_HAS_TIMEOUT during demux for
 FUTEX_WAIT

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@...ux.intel.com>
> Reported-by: Tim Smith <tsmith201104@...oo.com>
> Reported-by: Torsten Hilbrich <torsten.hilbrich@...unet.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...e.hu>
> CC: Eric Dumazet <eric.dumazet@...il.com>
> CC: John Kacur <jkacur@...hat.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.

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

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;
 


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