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lib: sbi: Set the state of a hart to START_PENDING after the hart is ready

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When a boot hart executes sbi_hsm_hart_start() to start a secondary hart,
next_arg1, next_addr and next_mode for the latter are stored in the scratch
area after the state has been set to SBI_HSM_STATE_START_PENDING.

The secondary hart waits in the loop with wfi() in sbi_hsm_hart_wait() at
that time. However, "wfi" instruction is not guaranteed to wait for an
interrupt to be received by the hart, it is just a hint for the CPU.
According to RISC-V Privileged Architectures spec. v20211203, even an
implementation of "wfi" as "nop" is legal.

So, the secondary might leave the loop in sbi_hsm_hart_wait() as soon as
its state has been set to SBI_HSM_STATE_START_PENDING, even if it got no
IPI or it got an IPI unrelated to sbi_hsm_hart_start(). This could lead to
the following race condition when booting Linux, for example:

  Boot hart (#0)                        Secondary hart (#1)
  runs Linux startup code               waits in sbi_hsm_hart_wait()

  sbi_ecall(SBI_EXT_HSM,
            SBI_EXT_HSM_HART_START,
            ...)
  enters sbi_hsm_hart_start()
  sets state of hart #1 to START_PENDING
                                        leaves sbi_hsm_hart_wait()
                                        runs to the end of init_warmboot()
                                        returns to scratch->next_addr
                                        (next_addr can be garbage here)

  sets next_addr, etc. for hart #1
  (no good: hart #1 has already left)

  sends IPI to hart #1
  (no good either)

If this happens, the secondary hart jumps to a wrong next_addr at the end
of init_warmboot(), which leads to a system hang or crash.

To reproduce the issue more reliably, one could add a delay in
sbi_hsm_hart_start() after setting the hart's state but before sending
IPI to that hart:

    hstate = atomic_cmpxchg(&hdata->state, SBI_HSM_STATE_STOPPED,
                            SBI_HSM_STATE_START_PENDING);
    ...
  + sbi_timer_mdelay(10);
    init_count = sbi_init_count(hartid);
    rscratch->next_arg1 = arg1;
    rscratch->next_addr = saddr;

The issue can be reproduced, for example, in a QEMU VM with '-machine virt'
and 2 or more CPUs, with Linux as the guest OS.

This patch moves writing of next_arg1, next_addr and next_mode for the
secondary hart before setting its state to SBI_HSM_STATE_START_PENDING.

In theory, it is possible that two or more harts enter sbi_hsm_hart_start()
for the same target hart simultaneously. To make sure the current hart has
exclusive access to the scratch area of the target hart at that point, a
per-hart 'start_ticket' is used. It is initially 0. The current hart tries
to acquire the ticket first (set it to 1) at the beginning of
sbi_hsm_hart_start() and only proceeds if it has successfully acquired it.

The target hart reads next_addr, etc., and then the releases the ticket
(sets it to 0) before calling sbi_hart_switch_mode(). This way, even if
some other hart manages to enter sbi_hsm_hart_start() after the ticket has
been released but before the target hart jumps to next_addr, it will not
cause problems.

atomic_cmpxchg() already has "acquire" semantics, among other things, so
no additional barriers are needed in hsm_start_ticket_acquire(). No hart
can perform or observe the update of *rscratch before setting of
'start_ticket' to 1.

atomic_write() only imposes ordering of writes, so an explicit barrier is
needed in hsm_start_ticket_release() to ensure its "release" semantics.
This guarantees that reads of scratch->next_addr, etc., in
sbi_hsm_hart_start_finish() cannot happen after 'start_ticket' has been
released.

Signed-off-by: Evgenii Shatokhin <e.shatokhin@yadro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
This commit is contained in:
Evgenii Shatokhin 2023-03-06 00:22:46 +03:00 • committed by Anup Patel
parent d56049e299
commit e8e9ed3790
1 changed files with 71 additions and 20 deletions
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91
lib/sbi/sbi_hsm.c
View file

@ -44,6 +44,7 @@ struct sbi_hsm_data {
unsigned long suspend_type;
unsigned long saved_mie;
unsigned long saved_mip;
atomic_t start_ticket;
};
bool sbi_hsm_hart_change_state(struct sbi_scratch *scratch, long oldstate,
@ -75,6 +76,32 @@ int sbi_hsm_hart_get_state(const struct sbi_domain *dom, u32 hartid)
return __sbi_hsm_hart_get_state(hartid);
}
/*
* Try to acquire the ticket for the given target hart to make sure only
* one hart prepares the start of the target hart.
* Returns true if the ticket has been acquired, false otherwise.
*
* The function has "acquire" semantics: no memory operations following it
* in the current hart can be seen before it by other harts.
* atomic_cmpxchg() provides the memory barriers needed for that.
*/
static bool hsm_start_ticket_acquire(struct sbi_hsm_data *hdata)
{
return (atomic_cmpxchg(&hdata->start_ticket, 0, 1) == 0);
}
/*
* Release the ticket for the given target hart.
*
* The function has "release" semantics: no memory operations preceding it
* in the current hart can be seen after it by other harts.
*/
static void hsm_start_ticket_release(struct sbi_hsm_data *hdata)
{
RISCV_FENCE(rw, w);
atomic_write(&hdata->start_ticket, 0);
}
/**
* Get ulong HART mask for given HART base ID
* @param dom the domain to be used for output HART mask
@ -113,6 +140,9 @@ int sbi_hsm_hart_interruptible_mask(const struct sbi_domain *dom,
void __noreturn sbi_hsm_hart_start_finish(struct sbi_scratch *scratch,
u32 hartid)
{
unsigned long next_arg1;
unsigned long next_addr;
unsigned long next_mode;
struct sbi_hsm_data *hdata = sbi_scratch_offset_ptr(scratch,
hart_data_offset);
@ -120,8 +150,12 @@ void __noreturn sbi_hsm_hart_start_finish(struct sbi_scratch *scratch,
SBI_HSM_STATE_STARTED))
sbi_hart_hang();
sbi_hart_switch_mode(hartid, scratch->next_arg1, scratch->next_addr,
scratch->next_mode, false);
next_arg1 = scratch->next_arg1;
next_addr = scratch->next_addr;
next_mode = scratch->next_mode;
hsm_start_ticket_release(hdata);
sbi_hart_switch_mode(hartid, next_arg1, next_addr, next_mode, false);
}
static void sbi_hsm_hart_wait(struct sbi_scratch *scratch, u32 hartid)
@ -226,6 +260,7 @@ int sbi_hsm_init(struct sbi_scratch *scratch, u32 hartid, bool cold_boot)
(i == hartid) ?
SBI_HSM_STATE_START_PENDING :
SBI_HSM_STATE_STOPPED);
ATOMIC_INIT(&hdata->start_ticket, 0);
}
} else {
sbi_hsm_hart_wait(scratch, hartid);
@ -270,6 +305,7 @@ int sbi_hsm_hart_start(struct sbi_scratch *scratch,
unsigned int hstate;
struct sbi_scratch *rscratch;
struct sbi_hsm_data *hdata;
int rc;
/* For now, we only allow start mode to be S-mode or U-mode. */
if (smode != PRV_S && smode != PRV_U)
@ -283,17 +319,9 @@ int sbi_hsm_hart_start(struct sbi_scratch *scratch,
rscratch = sbi_hartid_to_scratch(hartid);
if (!rscratch)
return SBI_EINVAL;
hdata = sbi_scratch_offset_ptr(rscratch, hart_data_offset);
hstate = atomic_cmpxchg(&hdata->state, SBI_HSM_STATE_STOPPED,
SBI_HSM_STATE_START_PENDING);
if (hstate == SBI_HSM_STATE_STARTED)
return SBI_EALREADY;
/**
* if a hart is already transition to start or stop, another start call
* is considered as invalid request.
*/
if (hstate != SBI_HSM_STATE_STOPPED)
hdata = sbi_scratch_offset_ptr(rscratch, hart_data_offset);
if (!hsm_start_ticket_acquire(hdata))
return SBI_EINVAL;
init_count = sbi_init_count(hartid);
@ -301,16 +329,39 @@ int sbi_hsm_hart_start(struct sbi_scratch *scratch,
rscratch->next_addr = saddr;
rscratch->next_mode = smode;
if (hsm_device_has_hart_hotplug() ||
(hsm_device_has_hart_secondary_boot() && !init_count)) {
return hsm_device_hart_start(hartid, scratch->warmboot_addr);
} else {
int rc = sbi_ipi_raw_send(hartid);
if (rc)
return rc;
/*
* atomic_cmpxchg() is an implicit barrier. It makes sure that
* other harts see reading of init_count and writing to *rscratch
* before hdata->state is set to SBI_HSM_STATE_START_PENDING.
*/
hstate = atomic_cmpxchg(&hdata->state, SBI_HSM_STATE_STOPPED,
SBI_HSM_STATE_START_PENDING);
if (hstate == SBI_HSM_STATE_STARTED) {
rc = SBI_EALREADY;
goto err;
}
return 0;
/**
* if a hart is already transition to start or stop, another start call
* is considered as invalid request.
*/
if (hstate != SBI_HSM_STATE_STOPPED) {
rc = SBI_EINVAL;
goto err;
}
if (hsm_device_has_hart_hotplug() ||
(hsm_device_has_hart_secondary_boot() && !init_count)) {
rc = hsm_device_hart_start(hartid, scratch->warmboot_addr);
} else {
rc = sbi_ipi_raw_send(hartid);
}
if (!rc)
return 0;
err:
hsm_start_ticket_release(hdata);
return rc;
}
int sbi_hsm_hart_stop(struct sbi_scratch *scratch, bool exitnow)