Re: [PATCH] perf: Fix probing for PERF_FLAG_FD_CLOEXEC flag

From: David Ahern
Date: Thu Feb 19 2015 - 11:27:42 EST

On 2/19/15 9:17 AM, Adrian Hunter wrote:
Yes, I am sorry it is a pain. I don't know why I didn't add a comment
to the code :-(. Using -1 for the pid is a workaround to avoid gratuitous
jump label changes. If pid=0 is used and then a system-wide trace is done
with Intel PT, there will be a jump label change shortly after the tracing
starts. That means the running code gets changed, but Intel PT decoding
has to walk the code to reconstruct the trace - so errors result. There
will always be occasional jump label changes, but this avoids one that
would otherwise always happen.

I don't understand the response. Why can't pid == getpid() (ie., pid > 0) be used for this test? pid = -1 and pid = 0 are not needed. With pid > 0 cpu value does not matter so cpu = -1 can be used. Again this is just to determine if the kernel supports PERF_FLAG_FD_CLOEXEC. Existence of PT should not be involved here.

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