linux-kernel - Re: [PATCH 2/2] perf/core: fix bad use of igrab in kernel/event/core.c

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Date:   Thu, 19 Apr 2018 09:17:10 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] perf/core: fix bad use of igrab in
 kernel/event/core.c

On Tue, Apr 17, 2018 at 11:29:07PM -0700, Song Liu wrote:
> As Miklos reported and suggested:
> 
>   This pattern repeats two times in trace_uprobe.c and in
>   kernel/events/core.c as well:
> 
>       ret = kern_path(filename, LOOKUP_FOLLOW, &path);
>       if (ret)
>           goto fail_address_parse;
> 
>       inode = igrab(d_inode(path.dentry));
>       path_put(&path);
> 
>   And it's wrong.  You can only hold a reference to the inode if you
>   have an active ref to the superblock as well (which is normally
>   through path.mnt) or holding s_umount.

Oops. I must have snatched it from the uprobe code without thinking.

>   This way unmounting the containing filesystem while the tracepoint is
>   active will give you the "VFS: Busy inodes after unmount..." message
>   and a crash when the inode is finally put.
> 
>   Solution: store path instead of inode.
> 
> This patch fixes the issue in kernel/event/core.c.
> 
> NOTE: Based on my understanding, perf_addr_filter only supports intel_pt.

Coresight too, but that's probably even further away from what you have.

> However, my test system doesn't support address filtering (or I made a
> mistake?). Therefore, I have NOT tested this patch.

Check /sys/devices/intel_pt/caps/num_address_ranges, if it's non-zero,
it's supported.

> Could someone please help test it?

Yes:

Reviewed-and-tested-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>

The subject line needs a little love to be more like other perf commits, but
other than that, looks good.

Thanks!