ring-buffer: Do not trigger WARN_ON() due to a commit_overrun

In the Linux kernel, the following vulnerability has been resolved:

ring-buffer: Do not trigger WARN_ON() due to a commit_overrun

When reading a memory mapped buffer the reader page is just swapped out
with the last page written in the write buffer. If the reader page is the
same as the commit buffer (the buffer that is currently being written to)
it was assumed that it should never have missed events. If it does, it
triggers a WARN_ON_ONCE().

But there just happens to be one scenario where this can legitimately
happen. That is on a commit_overrun. A commit overrun is when an interrupt
preempts an event being written to the buffer and then the interrupt adds
so many new events that it fills and wraps the buffer back to the commit.
Any new events would then be dropped and be reported as "missed_events".

In this case, the next page to read is the commit buffer and after the
swap of the reader page, the reader page will be the commit buffer, but
this time there will be missed events and this triggers the following
warning:

------------[ cut here ]------------
WARNING: CPU: 2 PID: 1127 at kernel/trace/ring_buffer.c:7357 ring_buffer_map_get_reader+0x49a/0x780
Modules linked in: kvm_intel kvm irqbypass
CPU: 2 UID: 0 PID: 1127 Comm: trace-cmd Not tainted 6.15.0-rc7-test-00004-g478bc2824b45-dirty #564 PREEMPT
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:ring_buffer_map_get_reader+0x49a/0x780
Code: 00 00 00 48 89 fe 48 c1 ee 03 80 3c 2e 00 0f 85 ec 01 00 00 4d 3b a6 a8 00 00 00 0f 85 8a fd ff ff 48 85 c0 0f 84 55 fe ff ff <0f> 0b e9 4e fe ff ff be 08 00 00 00 4c 89 54 24 58 48 89 54 24 50
RSP: 0018:ffff888121787dc0 EFLAGS: 00010002
RAX: 00000000000006a2 RBX: ffff888100062800 RCX: ffffffff8190cb49
RDX: ffff888126934c00 RSI: 1ffff11020200a15 RDI: ffff8881010050a8
RBP: dffffc0000000000 R08: 0000000000000000 R09: ffffed1024d26982
R10: ffff888126934c17 R11: ffff8881010050a8 R12: ffff888126934c00
R13: ffff8881010050b8 R14: ffff888101005000 R15: ffff888126930008
FS: 00007f95c8cd7540(0000) GS:ffff8882b576e000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f95c8de4dc0 CR3: 0000000128452002 CR4: 0000000000172ef0
Call Trace:
<TASK>
? __pfx_ring_buffer_map_get_reader+0x10/0x10
tracing_buffers_ioctl+0x283/0x370
__x64_sys_ioctl+0x134/0x190
do_syscall_64+0x79/0x1c0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f95c8de48db
Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1c 48 8b 44 24 18 64 48 2b 04 25 28 00 00
RSP: 002b:00007ffe037ba110 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007ffe037bb2b0 RCX: 00007f95c8de48db
RDX: 0000000000000000 RSI: 0000000000005220 RDI: 0000000000000006
RBP: 00007ffe037ba180 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffe037bb6f8 R14: 00007f95c9065000 R15: 00005575c7492c90
</TASK>
irq event stamp: 5080
hardirqs last enabled at (5079): [<ffffffff83e0adb0>] _raw_spin_unlock_irqrestore+0x50/0x70
hardirqs last disabled at (5080): [<ffffffff83e0aa83>] _raw_spin_lock_irqsave+0x63/0x70
softirqs last enabled at (4182): [<ffffffff81516122>] handle_softirqs+0x552/0x710
softirqs last disabled at (4159): [<ffffffff815163f7>] __irq_exit_rcu+0x107/0x210
---[ end trace 0000000000000000 ]---

The above was triggered by running on a kernel with both lockdep and KASAN
as well as kmemleak enabled and executing the following command:

# perf record -o perf-test.dat -a -- trace-cmd record --nosplice -e all -p function hackbench 50

With perf interjecting a lot of interrupts and trace-cmd enabling all
events as well as function tracing, with lockdep, KASAN and kmemleak
enabled, it could cause an interrupt preempting an event being written to
add enough event
---truncated---