linux-kernel - Re: [PATCH] tracing/snapshot: resize spare buffer if size changed

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Message-ID: <20190628144814.748460d0@gandalf.local.home>
Date:   Fri, 28 Jun 2019 14:48:14 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Eiichi Tsukata <devel@...ukata.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing/snapshot: resize spare buffer if size changed

On Tue, 25 Jun 2019 10:29:10 +0900
Eiichi Tsukata <devel@...ukata.com> wrote:

> Current snapshot implementation swaps two ring_buffers even though their
> sizes are different from each other, that can cause an inconsistency
> between the contents of buffer_size_kb file and the current buffer size.
> 
> For example:
> 
>   # cat buffer_size_kb
>   7 (expanded: 1408)
>   # echo 1 > events/enable
>   # grep bytes per_cpu/cpu0/stats
>   bytes: 1441020
>   # echo 1 > snapshot             // current:1408, spare:1408
>   # echo 123 > buffer_size_kb     // current:123,  spare:1408
>   # echo 1 > snapshot             // current:1408, spare:123
>   # grep bytes per_cpu/cpu0/stats
>   bytes: 1443700
>   # cat buffer_size_kb
>   123                             // != current:1408
> 
> And also, a similar per-cpu case hits the following WARNING:
> 
> Reproducer:
> 
>   # echo 1 > per_cpu/cpu0/snapshot
>   # echo 123 > buffer_size_kb
>   # echo 1 > per_cpu/cpu0/snapshot
> 
> WARNING:
> 
>   WARNING: CPU: 0 PID: 1946 at kernel/trace/trace.c:1607 update_max_tr_single.part.0+0x2b8/0x380
>   Modules linked in:
>   CPU: 0 PID: 1946 Comm: bash Not tainted 5.2.0-rc6 #20
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-2.fc30 04/01/2014
>   RIP: 0010:update_max_tr_single.part.0+0x2b8/0x380
>   Code: ff e8 dc da f9 ff 0f 0b e9 88 fe ff ff e8 d0 da f9 ff 44 89 ee bf f5 ff ff ff e8 33 dc f9 ff 41 83 fd f5 74 96 e8 b8 da f9 ff <0f> 0b eb 8d e8 af da f9 ff 0f 0b e9 bf fd ff ff e8 a3 da f9 ff 48
>   RSP: 0018:ffff888063e4fca0 EFLAGS: 00010093
>   RAX: ffff888066214380 RBX: ffffffff99850fe0 RCX: ffffffff964298a8
>   RDX: 0000000000000000 RSI: 00000000fffffff5 RDI: 0000000000000005
>   RBP: 1ffff1100c7c9f96 R08: ffff888066214380 R09: ffffed100c7c9f9b
>   R10: ffffed100c7c9f9a R11: 0000000000000003 R12: 0000000000000000
>   R13: 00000000ffffffea R14: ffff888066214380 R15: ffffffff99851060
>   FS:  00007f9f8173c700(0000) GS:ffff88806d000000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000000714dc0 CR3: 0000000066fa6000 CR4: 00000000000006f0
>   Call Trace:
>    ? trace_array_printk_buf+0x140/0x140
>    ? __mutex_lock_slowpath+0x10/0x10
>    tracing_snapshot_write+0x4c8/0x7f0
>    ? trace_printk_init_buffers+0x60/0x60
>    ? selinux_file_permission+0x3b/0x540
>    ? tracer_preempt_off+0x38/0x506
>    ? trace_printk_init_buffers+0x60/0x60
>    __vfs_write+0x81/0x100
>    vfs_write+0x1e1/0x560
>    ksys_write+0x126/0x250
>    ? __ia32_sys_read+0xb0/0xb0
>    ? do_syscall_64+0x1f/0x390
>    do_syscall_64+0xc1/0x390
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> This patch adds resize_buffer_duplicate_size() to check if there is a
> difference between current/spare buffer sizes and resize a spare buffer
> if necessary.
> 
> Signed-off-by: Eiichi Tsukata <devel@...ukata.com>
>

Applied, thanks Eiichi!

-- Steve