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SubjectRe: [PATCH 9/9] habanalabs: allow multiple processes to open FD
On Sun, Jul 28, 2019 at 02:56:40PM +0300, Oded Gabbay wrote:
> On Sun, Jul 28, 2019 at 2:44 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Sun, Jul 28, 2019 at 02:28:18PM +0300, Oded Gabbay wrote:
> > > This patch removes the limitation of a single process that can open the
> > > device.
> > >
> > > Now, there is no limitation on the number of processes that can open the
> > > device and have a valid FD.
> > >
> > > However, only a single process can perform compute operations. This is
> > > enforced by allowing only a single process to have a compute context.
> > >
> > > Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
> > > ---
> > > drivers/misc/habanalabs/context.c | 100 +++++++++++++++------
> > > drivers/misc/habanalabs/device.c | 18 ++--
> > > drivers/misc/habanalabs/habanalabs.h | 1 -
> > > drivers/misc/habanalabs/habanalabs_drv.c | 8 --
> > > drivers/misc/habanalabs/habanalabs_ioctl.c | 7 +-
> > > 5 files changed, 85 insertions(+), 49 deletions(-)
> > >
> > > diff --git a/drivers/misc/habanalabs/context.c b/drivers/misc/habanalabs/context.c
> > > index 57bbe59da9b6..f64220fc3a55 100644
> > > --- a/drivers/misc/habanalabs/context.c
> > > +++ b/drivers/misc/habanalabs/context.c
> > > @@ -56,7 +56,7 @@ void hl_ctx_do_release(struct kref *ref)
> > > kfree(ctx);
> > > }
> > >
> > > -int hl_ctx_create(struct hl_device *hdev, struct hl_fpriv *hpriv)
> > > +static int hl_ctx_create(struct hl_device *hdev, struct hl_fpriv *hpriv)
> > > {
> > > struct hl_ctx_mgr *mgr = &hpriv->ctx_mgr;
> > > struct hl_ctx *ctx;
> > > @@ -89,9 +89,6 @@ int hl_ctx_create(struct hl_device *hdev, struct hl_fpriv *hpriv)
> > > /* TODO: remove for multiple contexts per process */
> > > hpriv->ctx = ctx;
> > >
> > > - /* TODO: remove the following line for multiple process support */
> > > - hdev->compute_ctx = ctx;
> > > -
> > > return 0;
> > >
> > > remove_from_idr:
> > > @@ -206,13 +203,22 @@ bool hl_ctx_is_valid(struct hl_fpriv *hpriv, bool requires_compute_ctx)
> > > int rc;
> > >
> > > /* First thing, to minimize latency impact, check if context exists.
> > > - * Also check if it matches the requirements. If so, exit immediately
> > > + * This is relevant for the "steady state", where a process context
> > > + * already exists, and we want to minimize the latency in command
> > > + * submissions. In that case, we want to see if we can quickly exit
> > > + * with a valid answer.
> > > + *
> > > + * If a context doesn't exists, we must grab the mutex. Otherwise,
> > > + * there can be nasty races in case of multi-threaded application.
> > > + *
> > > + * So, if the context exists and we don't need a compute context,
> > > + * that's fine. If it exists and the context we have is the compute
> > > + * context, that's also fine. Other then that, we can't check anything
> > > + * without the mutex.
> > > */
> > > - if (hpriv->ctx) {
> > > - if ((requires_compute_ctx) && (hdev->compute_ctx != hpriv->ctx))
> > > - return false;
> > > + if ((hpriv->ctx) && ((!requires_compute_ctx) ||
> > > + (hdev->compute_ctx == hpriv->ctx)))
> > > return true;
> > > - }
> > >
> > > mutex_lock(&hdev->lazy_ctx_creation_lock);
> > >
> > > @@ -222,35 +228,73 @@ bool hl_ctx_is_valid(struct hl_fpriv *hpriv, bool requires_compute_ctx)
> > > * creation of a context
> > > */
> > > if (hpriv->ctx) {
> > > - if ((requires_compute_ctx) && (hdev->compute_ctx != hpriv->ctx))
> > > + if ((!requires_compute_ctx) ||
> > > + (hdev->compute_ctx == hpriv->ctx))
> > > + goto unlock_mutex;
> > > +
> > > + if (hdev->compute_ctx) {
> > > valid = false;
> > > - goto unlock_mutex;
> > > - }
> > > + goto unlock_mutex;
> > > + }
> > >
> > > - /* If we already have a compute context, there is no point
> > > - * of creating one in case we are called from ioctl that needs
> > > - * a compute context
> > > - */
> > > - if ((hdev->compute_ctx) && (requires_compute_ctx)) {
> > > + /* If we reached here, it means we have a non-compute context,
> > > + * but there is no compute context on the device. Therefore,
> > > + * we can try to "upgrade" the existing context to a compute
> > > + * context
> > > + */
> > > + dev_dbg_ratelimited(hdev->dev,
> > > + "Non-compute context %d exists\n",
> > > + hpriv->ctx->asid);
> > > +
> > > + } else if ((hdev->compute_ctx) && (requires_compute_ctx)) {
> > > +
> > > + /* If we already have a compute context in the device, there is
> > > + * no point of creating one in case we are called from ioctl
> > > + * that needs a compute context
> > > + */
> > > dev_err(hdev->dev,
> > > "Can't create new compute context as one already exists\n");
> > > valid = false;
> > > goto unlock_mutex;
> > > - }
> > > + } else {
> > > + /* If we reached here it is because there isn't a context for
> > > + * the process AND there is no compute context or compute
> > > + * context wasn't required. In any case, must create a context
> > > + * for the process
> > > + */
> > >
> > > - rc = hl_ctx_create(hdev, hpriv);
> > > - if (rc) {
> > > - dev_err(hdev->dev, "Failed to create context %d\n", rc);
> > > - valid = false;
> > > - goto unlock_mutex;
> > > + rc = hl_ctx_create(hdev, hpriv);
> > > + if (rc) {
> > > + dev_err(hdev->dev, "Failed to create context %d\n", rc);
> > > + valid = false;
> > > + goto unlock_mutex;
> > > + }
> > > +
> > > + dev_dbg_ratelimited(hdev->dev, "Created context %d\n",
> > > + hpriv->ctx->asid);
> > > }
> > >
> > > - /* Device is IDLE at this point so it is legal to change PLLs.
> > > - * There is no need to check anything because if the PLL is
> > > - * already HIGH, the set function will return without doing
> > > - * anything
> > > + /* If we reached here then either we have a new context, or we can
> > > + * upgrade a non-compute context to a compute context. Do the upgrade
> > > + * only if the caller required a compute context
> > > */
> > > - hl_device_set_frequency(hdev, PLL_HIGH);
> > > + if (requires_compute_ctx) {
> > > + WARN(hdev->compute_ctx,
> > > + "Compute context exists but driver is setting a new one");
> >
> > This will trigger syzbot and will reboot machines that have
> > 'panic-on-warn' set (i.e. all cloud systems). So be _VERY_ careful
> > about this.
> >
> > If a user can trigger this, do not use WARN(), that's not what it is
> > for.
> >
> > thanks,
> >
> > greg k-h
>
> I see...
> I'll replace it with dev_crit, but I wanted to ask if you recommend to
> never use WARN in drivers ? Just use it in kernel core code ?

It should never be used anywhere, unless you are about to crash. You
should just properly fix things up, log the error, and move on. Same
goes for a driver as well as "core" kernel code.

If a user can trigger a WARN message, then that's a real big problem.
Again, think of 'panic-on-warn' systems.

If the hardware has hosed the system so bad that you can not do anything
else, just stop allowing access to the hardware. You shouldn't cause
the system to crash/reboot whenever possible.

thanks,

greg k-h

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