Commit 2a8db5ec authored by Conor Dooley's avatar Conor Dooley Committed by Palmer Dabbelt

RISC-V: Don't check text_mutex during stop_machine

We're currently using stop_machine() to update ftrace & kprobes, which
means that the thread that takes text_mutex during may not be the same
as the thread that eventually patches the code.  This isn't actually a
race because the lock is still held (preventing any other concurrent
accesses) and there is only one thread running during stop_machine(),
but it does trigger a lockdep failure.

This patch just elides the lockdep check during stop_machine.

Fixes: c15ac4fd ("riscv/ftrace: Add dynamic function tracer support")
Suggested-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Reported-by: default avatarChangbin Du <changbin.du@gmail.com>
Signed-off-by: default avatarPalmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: default avatarConor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230303143754.4005217-1-conor.dooley@microchip.comSigned-off-by: default avatarPalmer Dabbelt <palmer@rivosinc.com>
parent 76950340
......@@ -109,6 +109,6 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
#define ftrace_init_nop ftrace_init_nop
#endif
#endif
#endif /* CONFIG_DYNAMIC_FTRACE */
#endif /* _ASM_RISCV_FTRACE_H */
......@@ -9,4 +9,6 @@
int patch_text_nosync(void *addr, const void *insns, size_t len);
int patch_text(void *addr, u32 *insns, int ninsns);
extern int riscv_patch_in_stop_machine;
#endif /* _ASM_RISCV_PATCH_H */
......@@ -15,10 +15,19 @@
void ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
{
mutex_lock(&text_mutex);
/*
* The code sequences we use for ftrace can't be patched while the
* kernel is running, so we need to use stop_machine() to modify them
* for now. This doesn't play nice with text_mutex, we use this flag
* to elide the check.
*/
riscv_patch_in_stop_machine = true;
}
void ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
{
riscv_patch_in_stop_machine = false;
mutex_unlock(&text_mutex);
}
......@@ -107,9 +116,9 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
{
int out;
ftrace_arch_code_modify_prepare();
mutex_lock(&text_mutex);
out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
ftrace_arch_code_modify_post_process();
mutex_unlock(&text_mutex);
return out;
}
......
......@@ -11,6 +11,7 @@
#include <asm/kprobes.h>
#include <asm/cacheflush.h>
#include <asm/fixmap.h>
#include <asm/ftrace.h>
#include <asm/patch.h>
struct patch_insn {
......@@ -20,6 +21,8 @@ struct patch_insn {
atomic_t cpu_count;
};
int riscv_patch_in_stop_machine = false;
#ifdef CONFIG_MMU
/*
* The fix_to_virt(, idx) needs a const value (not a dynamic variable of
......@@ -60,7 +63,14 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
* Before reaching here, it was expected to lock the text_mutex
* already, so we don't need to give another lock here and could
* ensure that it was safe between each cores.
*
* We're currently using stop_machine() for ftrace & kprobes, and while
* that ensures text_mutex is held before installing the mappings it
* does not ensure text_mutex is held by the calling thread. That's
* safe but triggers a lockdep failure, so just elide it for that
* specific case.
*/
if (!riscv_patch_in_stop_machine)
lockdep_assert_held(&text_mutex);
if (across_pages)
......@@ -125,6 +135,7 @@ NOKPROBE_SYMBOL(patch_text_cb);
int patch_text(void *addr, u32 *insns, int ninsns)
{
int ret;
struct patch_insn patch = {
.addr = addr,
.insns = insns,
......@@ -132,7 +143,18 @@ int patch_text(void *addr, u32 *insns, int ninsns)
.cpu_count = ATOMIC_INIT(0),
};
return stop_machine_cpuslocked(patch_text_cb,
&patch, cpu_online_mask);
/*
* kprobes takes text_mutex, before calling patch_text(), but as we call
* calls stop_machine(), the lockdep assertion in patch_insn_write()
* gets confused by the context in which the lock is taken.
* Instead, ensure the lock is held before calling stop_machine(), and
* set riscv_patch_in_stop_machine to skip the check in
* patch_insn_write().
*/
lockdep_assert_held(&text_mutex);
riscv_patch_in_stop_machine = true;
ret = stop_machine_cpuslocked(patch_text_cb, &patch, cpu_online_mask);
riscv_patch_in_stop_machine = false;
return ret;
}
NOKPROBE_SYMBOL(patch_text);
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