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Subject: Re: svn commit: r273549 - head/sys/kern
From: =?utf-8?Q?Edward_Tomasz_Napiera=C5=82a?= <trasz@FreeBSD.org>
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Date: Thu, 23 Oct 2014 20:45:13 +0200
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References: <201410231535.s9NFZlbn002656@svn.freebsd.org>
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To: John Baldwin <jhb@freebsd.org>
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Dnia 23 pa=C5=BA 2014 o godz. 20:38 John Baldwin <jhb@freebsd.org> napisa=C5=
=82(a):

>> On Thursday, October 23, 2014 11:35:47 am Mateusz Guzik wrote:
>> Author: mjg
>> Date: Thu Oct 23 15:35:47 2014
>> New Revision: 273549
>> URL: https://svnweb.freebsd.org/changeset/base/273549
>>=20
>> Log:
>>  Avoid taking the lock in selfdfree when not needed.
>>=20
>> Modified:
>>  head/sys/kern/sys_generic.c
>>=20
>> Modified: head/sys/kern/sys_generic.c
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D
>> --- head/sys/kern/sys_generic.c    Thu Oct 23 15:16:40 2014    (r273548)
>> +++ head/sys/kern/sys_generic.c    Thu Oct 23 15:35:47 2014    (r273549)
>> @@ -1600,10 +1600,11 @@ static void
>> selfdfree(struct seltd *stp, struct selfd *sfp)
>> {
>>    STAILQ_REMOVE(&stp->st_selq, sfp, selfd, sf_link);
>> -    mtx_lock(sfp->sf_mtx);
>> -    if (sfp->sf_si)
>> +    if (sfp->sf_si !=3D NULL) {
>> +        mtx_lock(sfp->sf_mtx);
>>        TAILQ_REMOVE(&sfp->sf_si->si_tdlist, sfp, sf_threads);
>> -    mtx_unlock(sfp->sf_mtx);
>> +        mtx_unlock(sfp->sf_mtx);
>> +    }
>>    uma_zfree(selfd_zone, sfp);
>=20
> How do you ensure that the value you read for sf_si here is up to date?  I=
n=20
> particular, if a thread is selecting on multiple fds and one awakens it,
> another fd can invoke selwakeup() while the thread is in seltdclear().
> In that case, you might see a stale value of sf_si and not realize it is=20=

> cleared by the selwakeup() after you get the lock and you will invoke
> TAILQ_REMOVE an extra time.

FWIW, I've just hit a panic in selfdfree().