Subject: Re: linux-next test error: BUG: using smp_processor_id() in preemptible [ADDR] code: syz-fuzzer/6792

From: Ido Schimmel <[email-masked]>
Date: Fri, 12 Jun 2020 15:43:40 +0300

On Tue, Jun 02, 2020 at 06:11:29PM +0530, Ritesh Harjani wrote:
> #syz test:
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
> 0e21d4620dd047da7952f44a2e1ac777ded2d57e

> >From cc1cf67d99d5fa61db0651c89c288df31bad6b8e Mon Sep 17 00:00:00 2001
> From: Ritesh Harjani <[email-masked]>
> Date: Tue, 2 Jun 2020 17:54:12 +0530
> Subject: [PATCH 1/1] ext4: mballoc: Use raw_cpu_ptr in case if preemption is enabled
> 
> It doesn't matter really in ext4_mb_new_blocks() about whether the code
> is rescheduled on any other cpu due to preemption. Because we care
> about discard_pa_seq only when the block allocation fails and then too
> we add the seq counter of all the cpus against the initial sampled one
> to check if anyone has freed any blocks while we were doing allocation.
> 
> So just use raw_cpu_ptr to not trigger this BUG.
> 
> BUG: using smp_processor_id() in preemptible [00000000] code: syz-fuzzer/6927
> caller is ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
> CPU: 1 PID: 6927 Comm: syz-fuzzer Not tainted 5.7.0-next-20200602-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x18f/0x20d lib/dump_stack.c:118
>  check_preemption_disabled+0x20d/0x220 lib/smp_processor_id.c:48
>  ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
>  ext4_ext_map_blocks+0x201b/0x33e0 fs/ext4/extents.c:4244
>  ext4_map_blocks+0x4cb/0x1640 fs/ext4/inode.c:626
>  ext4_getblk+0xad/0x520 fs/ext4/inode.c:833
>  ext4_bread+0x7c/0x380 fs/ext4/inode.c:883
>  ext4_append+0x153/0x360 fs/ext4/namei.c:67
>  ext4_init_new_dir fs/ext4/namei.c:2757 [inline]
>  ext4_mkdir+0x5e0/0xdf0 fs/ext4/namei.c:2802
>  vfs_mkdir+0x419/0x690 fs/namei.c:3632
>  do_mkdirat+0x21e/0x280 fs/namei.c:3655
>  do_syscall_64+0x60/0xe0 arch/x86/entry/common.c:359
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Signed-off-by: Ritesh Harjani <[email-masked]>
> Reported-by: [email-masked]

Hi,

Are you going to submit this patch formally? Without it I'm constantly
seeing the above splat.

Thanks

> ---
>  fs/ext4/mballoc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a9083113a8c0..b79b32dbe3ea 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4708,7 +4708,7 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
>  	}
>  
>  	ac->ac_op = EXT4_MB_HISTORY_PREALLOC;
> -	seq = *this_cpu_ptr(&discard_pa_seq);
> +	seq = *raw_cpu_ptr(&discard_pa_seq);
>  	if (!ext4_mb_use_preallocated(ac)) {
>  		ac->ac_op = EXT4_MB_HISTORY_ALLOC;
>  		ext4_mb_normalize_request(ac, ar);
> -- 
> 2.21.3
>