function_graph: Fix up ftrace_graph_ret_addr()

Yang Li sent a patch to fix the kerneldoc of ftrace_graph_ret_addr().
While reviewing it, I realized that the comments in the entire function
header needed a rewrite. When doing that, I realized that @idx parameter
was being ignored. Every time this was called by the unwinder, it would
start the loop at the top of the shadow stack and look for the matching
stack pointer. When it found it, it would return it. When the unwinder
asked for the next function, it would search from the beginning again.

In reality, it should start from where it left off. That was the reason
for the @idx parameter in the first place. The first time the unwinder
calls this function, the @idx pointer would contain zero. That would mean
to start from the top of the stack. The function was supposed to update
the @idx with the index where it found the return address, so that the
next time the unwinder calls this function it doesn't have to search
through the previous addresses it found (making it O(n^2)!).

This speeds up the unwinder's use of ftrace_graph_ret_addr() by an order
of magnitude.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]/
Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Guo Ren <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: WANG Xuerui <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: "Naveen N. Rao" <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Reported-by: Yang Li <[email protected]>
Fixes: 7aa1eaef9f428 ("function_graph: Allow multiple users to attach to function graph")
Signed-off-by: Steven Rostedt (Google) <[email protected]>

0 files changed, 0 insertions, 0 deletions