Fix: timestamp_end field should include all events within sub-buffer
authorMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tue, 30 Apr 2019 15:23:44 +0000 (11:23 -0400)
committerMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tue, 30 Apr 2019 19:01:01 +0000 (15:01 -0400)
commit74e4d0d1536ebc5b3ba01dabcdea468614ced6f7
treeed5ffbff39f494d0a8e67132c6e3df15481cf076
parentc9b22e40b850709413adb7a61380115d6e616a10
Fix: timestamp_end field should include all events within sub-buffer

Fix for timestamp_end not including all events within sub-buffer. This
happens if a thread is preempted/interrupted for a long time between
reserve and commit (e.g. in the middle of a packet), which causes the
timestamp used for timestamp_end field of the packet header to be lower
than the timestamp of the last events in the buffer (those following the
event that was preempted/interrupted between reserve and commit).

The fix involves sampling the timestamp when doing the last space
reservation in a sub-buffer (which necessarily happens before doing the
delivery after its last commit). Save this timestamp temporarily in a
per-sub-buffer control area (we have exclusive access to that area until
we increment the commit counter).

Then, that timestamp value will be read when delivering the sub-buffer,
whichever event or switch happens to be the last to increment the commit
counter to perform delivery. The timestamp value can be read without
worrying about concurrent access, because at that point sub-buffer
delivery has exclusive access to the sub-buffer.

This ensures the timestamp_end value is always larger or equal to the
timestamp of the last event, always below or equal the timestamp_begin
of the following packet, and always below or equal the timestamp of the
first event in the following packet.

This changes the layout of the ring buffer shared memory area, so we
need to bump the LTTNG_UST_ABI version from 7.2 to 8.0, thus requiring
locked-step upgrade between liblttng-ust in applications, session
daemon, and consumer daemon. This fix therefore cannot be backported
to existing stable releases.

Fixes: #1183
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
include/lttng/ust-abi.h
libringbuffer/frontend_types.h
libringbuffer/ring_buffer_frontend.c
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