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Subject: Re: splice() rcu_sched self-detected stall on CPU
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <20200505185732.GP2869@paulmck-ThinkPad-P72>
Date:   Tue, 5 May 2020 15:00:34 -0400
Cc:     Al Viro <viro@ZenIV.linux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org
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References: <89F418A9-EB20-48CB-9AE0-52C700E6BB74@lca.pw>
 <20200505185732.GP2869@paulmck-ThinkPad-P72>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On May 5, 2020, at 2:57 PM, Paul E. McKenney <paulmck@kernel.org> =
wrote:
>=20
> On Mon, May 04, 2020 at 03:11:09PM -0400, Qian Cai wrote:
>> Running a syscall fuzzer inside a container on linux-next floods =
systems with soft lockups. It looks like stuck in this line at =
iov_iter_copy_from_user_atomic(), Thoughts?
>>=20
>> iterate_all_kinds(i, bytes, v,
>>                copyin((p +=3D v.iov_len) - v.iov_len, v.iov_base, =
v.iov_len),
>>                memcpy_from_page((p +=3D v.bv_len) - v.bv_len, =
v.bv_page,
>>                                 v.bv_offset, v.bv_len),
>>                memcpy((p +=3D v.iov_len) - v.iov_len, v.iov_base, =
v.iov_len)
>>        )
>=20
> If the size being copied is large enough, something like this might =
happen.
>=20
> Is this a CONFIG_PREEMPT=3Dn kernel?  And is the size passed in to

Yes, CONFIG_PREEMPT=3Dn.

> iov_iter_copy_from_user_atomic() quite large, given that this is =
generated
> by a fuzzer?  If so, one thing to try is to add cond_resched() in the
> iterate_bvec(), iterate_kvec(), and iterate_iovec() macros.

I=E2=80=99ll try that.=