| Message ID | 20220208204858.3406-2-ingo.rohloff@lauterbach.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | USB: usbfs: replace atomic64 accesses by spinlock | expand |
On Tue, Feb 08, 2022 at 09:48:58PM +0100, Ingo Rohloff wrote: > While the fromer code imposes a limit on the used memory, it might be over By "fromer", you probably mean "former". But that's an uncommon usage which might confuse people (I was confused when I first read it). It would be better to say "existing code". > pessimistic (even if this is unlikely). > > Example scenario: > 8 threads running in parallel, all entering > "usbfs_increase_memory_usage()" at the same time. > The atomic accesses in "usbfs_increase_memory_usage()" could be > serialized like this: > 8 x "atomic64_add" > 8 x "atomic64_read" > If the 8 x "atomic64_add" raise "usbfs_memory_usage" above the limit, > then all 8 calls of "usbfs_increase_memory_usage()" will return with > -ENOMEM. If you instead serialize over the whole access to > "usbfs_memory_usage" by using a spinlock, some of these calls will > succeed. > > Signed-off-by: Ingo Rohloff <ingo.rohloff@lauterbach.com> > --- > drivers/usb/core/devio.c | 35 ++++++++++++++++++++++++----------- > 1 file changed, 24 insertions(+), 11 deletions(-) > > diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c > index fa66e6e58792..522b170c42de 100644 > --- a/drivers/usb/core/devio.c > +++ b/drivers/usb/core/devio.c > @@ -139,30 +139,43 @@ MODULE_PARM_DESC(usbfs_memory_mb, > /* Hard limit, necessary to avoid arithmetic overflow */ > #define USBFS_XFER_MAX (UINT_MAX / 2 - 1000000) > > -static atomic64_t usbfs_memory_usage; /* Total memory currently allocated */ > +static DEFINE_SPINLOCK(usbfs_memory_usage_lock); > +static u64 usbfs_memory_usage; /* Total memory currently allocated */ > > /* Check whether it's okay to allocate more memory for a transfer */ > static int usbfs_increase_memory_usage(u64 amount) > { > - u64 lim; > + u64 lim, total_mem; > + unsigned long flags; > + int ret; > > - lim = READ_ONCE(usbfs_memory_mb); > + lim = usbfs_memory_mb; Don't eliminate this READ_ONCE(). It is needed because usbfs_memory_mb is a writable module parameter. If the value is changed by another process at the same time you read it, you could get a nonsense value unless you use READ_ONCE(). > lim <<= 20; > > - atomic64_add(amount, &usbfs_memory_usage); > - > - if (lim > 0 && atomic64_read(&usbfs_memory_usage) > lim) { > - atomic64_sub(amount, &usbfs_memory_usage); > - return -ENOMEM; > - } > + ret = 0; > + spin_lock_irqsave(&usbfs_memory_usage_lock, flags); > + total_mem = usbfs_memory_usage + amount; > + if (lim > 0 && total_mem > lim) > + ret = -ENOMEM; > + else > + usbfs_memory_usage = total_mem; > + spin_unlock_irqrestore(&usbfs_memory_usage_lock, flags); > > - return 0; > + return ret; > } > > /* Memory for a transfer is being deallocated */ > static void usbfs_decrease_memory_usage(u64 amount) > { > - atomic64_sub(amount, &usbfs_memory_usage); > + u64 total_mem; > + unsigned long flags; > + > + spin_lock_irqsave(&usbfs_memory_usage_lock, flags); > + total_mem = usbfs_memory_usage; > + if (amount > total_mem) > + amount = total_mem; > + usbfs_memory_usage = total_mem - amount; As a matter of personal taste, I prefer: if (amount > usbfs_memory_usage) usbfs_memory_usage = 0; else usbfs_memory_usage -= amount; > + spin_unlock_irqrestore(&usbfs_memory_usage_lock, flags); > } > > static int connected(struct usb_dev_state *ps) Alan Stern
diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c index fa66e6e58792..522b170c42de 100644 --- a/drivers/usb/core/devio.c +++ b/drivers/usb/core/devio.c @@ -139,30 +139,43 @@ MODULE_PARM_DESC(usbfs_memory_mb, /* Hard limit, necessary to avoid arithmetic overflow */ #define USBFS_XFER_MAX (UINT_MAX / 2 - 1000000) -static atomic64_t usbfs_memory_usage; /* Total memory currently allocated */ +static DEFINE_SPINLOCK(usbfs_memory_usage_lock); +static u64 usbfs_memory_usage; /* Total memory currently allocated */ /* Check whether it's okay to allocate more memory for a transfer */ static int usbfs_increase_memory_usage(u64 amount) { - u64 lim; + u64 lim, total_mem; + unsigned long flags; + int ret; - lim = READ_ONCE(usbfs_memory_mb); + lim = usbfs_memory_mb; lim <<= 20; - atomic64_add(amount, &usbfs_memory_usage); - - if (lim > 0 && atomic64_read(&usbfs_memory_usage) > lim) { - atomic64_sub(amount, &usbfs_memory_usage); - return -ENOMEM; - } + ret = 0; + spin_lock_irqsave(&usbfs_memory_usage_lock, flags); + total_mem = usbfs_memory_usage + amount; + if (lim > 0 && total_mem > lim) + ret = -ENOMEM; + else + usbfs_memory_usage = total_mem; + spin_unlock_irqrestore(&usbfs_memory_usage_lock, flags); - return 0; + return ret; } /* Memory for a transfer is being deallocated */ static void usbfs_decrease_memory_usage(u64 amount) { - atomic64_sub(amount, &usbfs_memory_usage); + u64 total_mem; + unsigned long flags; + + spin_lock_irqsave(&usbfs_memory_usage_lock, flags); + total_mem = usbfs_memory_usage; + if (amount > total_mem) + amount = total_mem; + usbfs_memory_usage = total_mem - amount; + spin_unlock_irqrestore(&usbfs_memory_usage_lock, flags); } static int connected(struct usb_dev_state *ps)
While the fromer code imposes a limit on the used memory, it might be over pessimistic (even if this is unlikely). Example scenario: 8 threads running in parallel, all entering "usbfs_increase_memory_usage()" at the same time. The atomic accesses in "usbfs_increase_memory_usage()" could be serialized like this: 8 x "atomic64_add" 8 x "atomic64_read" If the 8 x "atomic64_add" raise "usbfs_memory_usage" above the limit, then all 8 calls of "usbfs_increase_memory_usage()" will return with -ENOMEM. If you instead serialize over the whole access to "usbfs_memory_usage" by using a spinlock, some of these calls will succeed. Signed-off-by: Ingo Rohloff <ingo.rohloff@lauterbach.com> --- drivers/usb/core/devio.c | 35 ++++++++++++++++++++++++----------- 1 file changed, 24 insertions(+), 11 deletions(-)